The present invention relates to a heat exchanger having a passage pipe through which thermal medium passes, and heat is exchanged between thermal medium and an object located outside of the passage pipe.
A conventional heat exchanger is known in which a passage pipe is arranged to sandwich a heat emitting object from both sides, and heat is emitted from the heat emitting object such as semiconductor module including a semiconductor element. In such heat exchanger, the heat emitting objects and the passage pipes are alternately layered with each other. The layered passage pipes are communicated with each other through a communication portion, and cooling medium flows each of the passage pipes.
In this kind of heat exchanger, in order to improve the heat exchanging property, a separating portion is arranged in the passage pipe so that two stairs of thermal medium passage are defined in the single passage pipe in a thickness direction. Further, an inner fin is arranged in each of the two stairs of thermal medium passage (refer to JP-A-2005-191527, for example).
By the way, in this kind of heat exchanger, because thermal medium is distributed into the passage pipes from the communication portion, a flowing speed of thermal medium in the passage pipe becomes slow. In order to improve the heat exchanging property in such low flow rate area in the passage pipe, a wave fin having a function of promoting a mixing of thermal medium in the passage pipe is used as the inner fin. A plurality of the wave fins are layered in the passage pipe in the thickness direction (refer to JP-A-2010-10418, for example).
Further, a method of promoting a turbulent flow is conventionally used so as to improve a heat transmitting ratio of the inner fin. For example, an offset fin is used as the inner fin, or a projection or slit is defined on a side face of the inner fin.
By the way, the turbulent flow promoting by the inner fin is effective in a case where a flowing speed of thermal medium in the passage pipe is fast, that is whose Reynolds number is larger than 1000 in which a flow of thermal medium is in a transient area or turbulent flow area. Further, as thermal medium flows faster, the turbulent flow is promoted so as to achieve the improvement of the property, but a flow resistance of thermal medium becomes large.
Further, in a case where the heat exchanger is used for cooling an inverter of a hybrid car, if a pump for circulating thermal medium in an inverter cooling circuit is made smaller, if a flow rate of the pump is made smaller, or if thermal medium passages are located parallel with each other, a flow rate of thermal medium passing through the passage pipe of the heat exchanger becomes small.
If the flow rate of thermal medium is small, the effect of promoting the turbulent flow cannot be obtained. Specifically, for example, when the offset fin is used as the inner fin, thermal medium flow along a wall face of the fin without colliding to the wall face, so that the expected effect of improving the property (R/ΔP) cannot be obtained.
That is, although it is known that the offset fin or pin fin is effective for promoting the turbulent flow, a property improving effect sufficiently large for canceling the increasing of the flow resistance cannot be obtained even using such fin when the flow rate of thermal medium in the passage pipe is small.